Method for a lithographic apparatus

Abstract

A method of increasing a depth of focus of a lithographic apparatus is disclosed. The method includes forming diffracted beams of radiation using a patterning device pattern; and transforming a phase-wavefront of a portion of the diffracted beams into a first phase-wavefront having a first focal plane for the lithographic apparatus, and a second phase-wavefront having a second, different focal plane, wherein the transforming comprises: subjecting a phase of a first portion of a first diffracted beam and a phase of a corresponding first portion of a second diffracted beam to a phase change which results in an at least partial formation of the first phase-wavefront, and subjecting a phase of a second portion of the first diffracted beam and a phase of a corresponding second portion of the second diffracted beam to a phase change which results in an at least partial formation of the second phase-wavefront.

1. A method of increasing a depth of focus of at least a part of a pattern feature imaged by a lithographic apparatus, the method comprising:
 illuminating a patterning device pattern, provided by a patterning device, with a radiation beam, the patterning device pattern comprising a pattern feature that diffracts the radiation beam to form a plurality of diffracted beams of radiation; 
 illuminating a phase modulation element with the diffracted beams of radiation emanating from the patterning device, and using the phase modulation element to control the phase of at least a portion of radiation constituting each of the diffracted beams of radiation to form a first phase-wavefront having a first focal plane for the lithographic apparatus, and form a second phase-wavefront having a second focal plane for the lithographic apparatus, the first and second focal planes being offset relative to one another gong an optical axis of the lithographic apparatus, wherein controlling the phase of the radiation constituting at least a portion of the diffracted beams of radiation comprises: 
 controlling the phase of a first portion of a first diffracted beam of radiation and a corresponding first portion of a second diffracted beam of radiation so that the first portions of the first and second diffracted beams of radiation are subjected to a first phase change which results in an at least partial formation of the first phase-wavefront, and 
 controlling the phase of a second portion of the first diffracted beam of radiation and a corresponding second portion of the second diffracted beam of radiation so that the second portions of the first and second diffracted beams of radiation are subjected to a second phase change which results in an at least partial formation of the second phase-wavefront. 
 
     
     
       2. The method of  claim 1 , wherein the first portions of the first and second diffracted beams of radiation and the second portions of the first and second diffracted beams of radiation are corresponding in that they have the same relative position in each respective diffracted beam of radiation. 
     
     
       3. The method of  claim 1 , wherein the first portion of the first or second diffracted beam of radiation and the second portion of the first or second diffracted beam of radiation define substantially equal areas when projected onto the phase modulation element. 
     
     
       4. The method of  claim 1 , wherein, when projected onto the phase modulation element, the first and second portions of the first or second diffracted beam of radiation meet at a center of the respective first or second diffracted beam of radiation. 
     
     
       5. The method of  claim 1 , wherein alternate and/or adjacent portions of each diffracted beam of radiation are subjected to a phase change which results in the at least partial formation of alternate phase-wavefronts. 
     
     
       6. The method of  claim 1 , wherein a phase of eight, sixteen or thirty two different portions of each diffracted beam of radiation is controlled. 
     
     
       7. The method of  claim 1 , wherein the first phase change together defines a radial phase distribution, or the second phase change together defines a radial phase distribution. 
     
     
       8. The method of  claim 1 , wherein:
 the first phase-wavefront and second phase-wavefront each have a different positive degree of curvature; or 
 the first phase-wavefront and second phase-wavefront each have a different negative degree of curvature; or 
 the first phase-wavefront has a positive degree of curvature and the second phase-wavefront has a negative degree of curvature. 
 
     
     
       9. The method of  claim 1 , wherein a phase change of the first portion of the first or second diffracted beam of radiation is substantially equal and opposite to a phase change of the second portion of the first or second diffracted beam of radiation. 
     
     
       10. The method of  claim 1 , wherein there is little or no overlap between the first and second diffracted beams of radiation when incident upon the phase modulation element, or wherein there is little or no overlap between the plurality of diffracted beams of radiation when incident upon the phase modulation element. 
     
     
       11. The method of  claim 1 , wherein the phase modulation element comprises a controllable region. 
     
     
       12. The method of  claim 11 , wherein the controllable region is controllable to change a refractive index of the controllable region. 
     
     
       13. The method of  claim 11 , wherein the controllable region is controllable by selectively heating the controllable region. 
     
     
       14. The method of  claim 11 , wherein the controllable region is controllable by selectively controlling a shape, position or orientation of the controllable region. 
     
     
       15. The method of  claim 1 , wherein the phase modulation element is located at or adjacent to a pupil plane of the lithographic apparatus. 
     
     
       16. The method of  claim 1 , wherein the first phase change is a zero phase change or the second phase change is a zero phase change. 
     
     
       17. A method of increasing a depth of focus of at least a part of a pattern feature imaged by a lithographic apparatus, the method comprising:
 forming diffracted beams of radiation by illuminating a patterning device pattern with a radiation beam, the patterning device pattern comprising a pattern feature that diffracts the radiation beam; and 
 illuminating a phase modulation element with the diffracted beams of radiation, and transforming a phase-wavefront of a portion of the diffracted beams of radiation into a first phase-wavefront having a first focal plane (and a first depth of focus) for the lithographic apparatus, and a second phase-wavefront having a second focal plane (and a second depth of focus) for the lithographic apparatus, the first and second focal planes (and first and second depths of focus) being offset relative to one another along an optical axis of the lithographic apparatus, 
 wherein the transforming comprises: 
 subjecting a phase of a first portion of a first diffracted beam of radiation and a phase of a corresponding first portion of a second diffracted beam of radiation to a phase change which results in an at least partial formation of the first phase-wavefront, and 
 subjecting a phase of a second portion of the first diffracted beam of radiation and a phase of a corresponding second portion of the second diffracted beam of radiation to a phase change which results in an at least partial formation of the second phase-wavefront.